Color cathode ray tube faceplates typically comprise a dished viewing portion having a concave inner surface upon which a phosphor screen is deposited. The screen comprises a mosaic of intercalated patterns of red-emissive, blue-emissive and green-emissive phosphor elements. The patterns of elements are deposited in succession, each by a series of operations which includes the application of a coating of an aqueous phosphor slurry to the faceplate inner surface. This invention concerns an improved process for applying such phosphor slurry coatings to the inner viewing surface of a color cathode ray tube faceplate. The phosphor slurries involved typically include a photosensitized organic binder and a suspended particulate phosphor material having a predetermined particle size distribution. The organic binder, typically PVA (polyvinyl alcohol), its sensitizer, and the phosphor material are commonly collectively termed the slurry "solids."
All known methods for disposing phosphor slurry coatings on color cathode ray tube faceplates involve an operation wherein a quantity of phosphor slurry is suffused or spread across the faceplate surface to be coated. This suffusion operation generally involves the application of a puddle of slurry to the surface to be coated, followed by one or more operations which cause the puddle of slurry to be spread across all areas of the surface. The excess slurry is then removed. Finally, a "levigation" operation is employed by which the suffused coating is leveled and thinned down to a predetermined thickness; typically this is accomplished by very rapidly spinning the faceplate, or in the disclosure of U.S. Pat. No. 3,700,444, by inverting the faceplate.
A number of important requirements are imposed upon the process by which phosphor slurry coatings are applied to a color cathode ray tube faceplate. Any process developed for applying phosphor slurry coatings to color CRT faceplates desirably should meet all these requirements, however, no known process has achieved all of these requirements.
Perhaps the most important requirement is that the coatings formed be uniform in weight throughout the viewing area of a given faceplate, and uniform from faceplate-to-faceplate during extended periods of factory production. It is also of utmost importance that the coatings be relatively thin, and yet have a sufficiently high phosphor particle density and phosphor coating weight that the cathode ray tube images ultimately produced will have maximum brightness. As used herein, the term "phosphor coating weight" means the weight per unit area of coated phosphor material, i.e., absent the binder and water. The term "phosphor particle density" or "phosphor density" refers to the weight of phosphor material per unit of volume, i.e., to how tightly the phosphor particles are packed. Phosphor density and phosphor coating weight are both important determinants of image brightness.
Any severe non-uniformities in the screen, such as result from radical variations in coating thickness, radial "streaks"tangential "sags", etc. which would be visible in the reproduced images must, of course, be suppressed to a tolerable level.
It is desirable also, in the interest of achieving maximum brightness in the reproduced images, that the phosphor particle size distribution in the resultant phosphor slurry coatings be in accordance with a specified predetermined particle size distribution. This holds not only over the viewing area of a given faceplate, but from faceplate-to-faceplate in factory production.
Further, for economic reasons it is desirable that the application of phosphor slurry coatings to color cathode ray tube faceplates be achieved in as brief an interval as possible and with a minimum number of work stations, and that the least possible expense be incurred in capital equipment and unit labor cost. It is also desirable that in the application of successive phosphor slurry coatings, later-deposited coatings do not contaminate earlier-deposited coatings.
A number of prior art methods for applying phosphor slurry coatings have been developed, however, none has been completely successful in meeting all the above-stated requirements. The commercially most common methods involve applying a puddle of slurry onto the concave inner surface of a faceplate while the faceplate is in a face-down position. The puddle is then suffused across the surface to be coated by tilting and rotating the panel to cause the puddle to move across all areas of the entire surface to be coated, or alternatively, by spinning the panel to cause the puddle to spread by centrifugal force across the faceplate inner surface. For example, see RCA Review, Vol. 16, pp. 122-139 (March, 1955) and U.S. Pat. Nos. 2,902,973; 3,319,759; 3,376,153; 3,364,054; 3,467,059; and 3,700,444.
Conventional faceplates have a rearwardly extending flange which contains the puddle. This invention is applicable to such conventional faceplates and color cathode ray tube faceplates in general, but is perhaps especially suited for use with a color cathode ray tube faceplate having a dished viewing portion but no flange, as will be described below.
These "puddling" methods are predicated on the principle that in order to achieve the high phosphor coating weights in the resultant phosphor slurry coatings, now deemed so necessary for high brightness image reproduction, a protracted time interval must be allotted for the phosphor particles, particularly the heaviest (and largest) particles, to settle out onto the surface to be coated. It is noteworthy that the alleged teaching of U.S. Pat. No. 3,653,941, self-touted to be an improvement on the methods of the prior art, causes the slurry to spiral inwardly and outwardly over the surface to be coated such that "additional time is allowed for particles in the slurry to settle upon the surface." In both the tilt-and-rotate method and the spin method of slurry coating, sufficient time is allotted for this sedimentation or settling out process to occur. The allotted sedimentation interval may, for example, be 60-80 seconds. The necessity for the provision of an adequate settling-out interval, however, results in an undesirably long duration coating operation.
Further, by the fact of the settling out of the phosphor particles from the phosphor slurry, the excess slurry which is dumped or spun from the panel cannot be reused without its first being reconstituted. This is because of the settling out of the phosphor particles from the slurry. For example, in a typical prior art process the dispensed slurry might have about 30% phosphor content (by weight), as dispensed. However, due to the settling out of the phosphor particles, the excess slurry will contain phosphor material in a substantially lower percentage than 30%. In order to bring the phosphor content back to predetermined value, and to maintain a predetermined phosphor-to-binder ratio, a "make-up" or "replenish" slurry, must be added to the collected slurry. Such a make-up slurry might have, e.g., a phosphor content of 45%, and will also have an increased phosphor-to-binder ratio. The actual phosphor contents and the phosphor-to-binder ratio are quite variable and must be monitored and controlled, a technically difficult task necessitating exacting measurements of phosphor content in the slurry and of slurry viscosity. It should be noted that whereas it is important to know and control the phosphor-to-binder ratio at all times during production, because of the difficulty and inconvenience of doing so, it is not industry practice to measure the phosphor-to-binder ratio in a slurry directly. Rather, phosphor content and slurry viscosity are periodically measured. Since the viscosity of a slurry is largely a function of temperature and relative content of water and binder in the slurry, if the slurry viscosity and phosphor content are maintained, it can be assumed that the phosphor-to-binder ratio is constant.
Due to the extreme difficulty in maintaining uniformity in the particle size distribution from faceplate-to-faceplate in factory production, control of particle size distribution in the dispensed phosphor slurry is generally neglected. As a result, the particle size distribution in such coatings will inevitably vary with time during a production run, resulting in the production of tubes having varying phosphor coating properties and hence varying brightness capabilities.
Further, any shifting in the particle size distribution toward lighter average particle sizes, when such a shift occurs during production, may result in aggravated color contamination in the reproduced images. This comes about because there are not one, but three, phosphor slurry coatings which are applied in succession to the inner surface of a faceplate. When the second layer is deposited upon the first layer, the finer particles in the second phosphor slurry coating tend to settle into crevices in the first layer and remain there after processing of the faceplate is completed. Similarly, upon application of the third phosphor slurry coating, the finer particles in the third phosphor slurry tend to settle into and permanently remain in the crevices in the first and second slurry coatings. Upon excitation by electron bombardment in the end-product tube, the contaminating phosphor particles emit light which contaminates or desaturates the true color light output.
The afore-described prior art methods, in general, involve a coating leveling or "levigation" process in which the faceplate is spun at high speeds to thin down and level the coatings of phosphor slurry which have been suffused across the faceplate inner surface. By the nature of the puddling-type processes, a relatively thick layer of phosphor slurry is deposited upon the faceplate surface. In order to thin down this thick layer to an acceptable coating thickness, the panel must be spun at an undesirably high speed (e.g. 250-300 RPM) for undesirably long interval, e.g., 15-20 seconds. It has been found that if a faceplate is spun at an excessively high speed during the leveling operation, or is spun at a more moderate speed for an excessively long interval, radial streaks or spokes are formed in the coatings. The radial streaks in the coating are caused by irregularities in the glass faceplate surface, or phosphor or black grille patterns on the faceplate surface, which interrupt the phosphor slurry being impelled radially outwardly by the centrifugal forces of rotation.
In the afore-described process in which the faceplate is spun to suffuse the dispensed slurry across the faceplate inner surface, the excess slurry is typically not removed by inverting the panel, as is common in tilt-type processes. Rather, the excess slurry is removed by spinning the faceplate at very high speeds to drive the excess slurry to the perimeter of the viewing surface and up and over the inner walls of the faceplate flange whereupon it is thrown from the faceplate by centrifugal force. Such high spin speeds have been found to cause radial streaking in the coatings formed.
Attempts have been made to develop commercially viable slurry coating processes which would not require the sedimentation principle and thus which would not suffer from the aforediscussed shortcomings of sedimentation processes -- e.g., see the FIG. 9 embodiment of U.S. Pat. No. 3,700,444. No non-sedimentation slurry process is known, however, prior to this invention, which results in the deposition of slurry coatings having commercially acceptable phosphor coating weights and thus commercially acceptable image brightness in the reproduced CRT images. It has heretofore been thought to be impractical, if not impossible, to deposit slurry coating of commercially adequate phosphor coating weight by any non-sedimentation process.
In the tilt-type process excess slurry is typically dumped from a corner of the faceplate. This introduces yet another shortcoming of the prior art tilt-type puddling process. By the fact of dumping from a corner of the faceplate, the resultant phosphor coatings are found to be heavier in the region of the faceplate where dumping is effected than in other regions of the faceplate. By way of example, variations in coating weight from corner to corner of a faceplate of .+-.10% are common in the practice of the tilt-type prior art process. The result is an uneven brightness in the images produced by the end-product tube.
It is known in the commercial manufacture of black grilles for color cathode ray tubes of the black matrix type to apply a uniform layer of a graphite material to the inner surface of a color cathode ray tube faceplate by flowing a graphite solution onto the faceplate while it is being rotated in a substantially vertical attitude. Such a process is disclosed in U.S. Pat. No. 3,652,323. A similar process is used earlier in the grill-making operation to apply a photosensitized PVA coating under the graphite layer. The graphite and PVA coating processes have little or no relevance, however, to the invention described and claimed herein.